1. Field of Invention
The present invention relates to a material hardness and tension and/or compression measuring device, and more specifically to a penetration hardness tester, which allows the user to measure, to high precision, the hardness of materials using many types of hardness tests.
2. Description of Related Arts
Well-known to those in the field, a lab needs many kinds of material testing machines, such as a Rockwell hardness tester, a Brinell hardness tester, a Vickers hardness tester, a Micro hardness tester and a Knoop hardness tester, as well as tension and compression testing machines. All manufacturers want to make a machine that can perform the same functions as many of if not all of the above machines, so that they can reduce machines quantities and costs. Until now, this is very difficult. All machines need to apply and measure precise and accurate forces and each machine has different amounts of force. For example, a Rockwell tester needs 10 kg force for minor forces and 150 kg for major forces, the Brinell tester needs a maximum force of 3000 kg, and micro Vickers tester needs precisely 0.1 kg force. Until now, a machine that can apply such a large range of forces accurately and precisely is not possible.
Even in a narrow range, machines have difficulty applying precise and accurate forces. For example, a Rockwell test requires 150 kg force. Using a 150 kg deadweight is too heavy for a lab machine, so designers use a 15 kg deadweight to apply a 150 kg force through levers. Because these mechanical parts will wear down over time, the force will eventually be inaccurate.
After the indenter penetrates to some depth or displacement in the test specimen, a measurement is made of the displacement. In prior art penetration hardness testers, there are moving mechanical parts which move relative to each other located between the actual displacement and measured displacement. Such relative mechanical movement can contribute to sources of friction or lost (non-recoverable) displacement between the point of displacement measurement and the test specimens so as to impair the repeated accuracy of the hardness test.
The use of deadweight testers and their mechanical impreciseness over time has led to the use of feedback control closed loop systems including a load cell as part of the means to measure the application of force to the test specimen.
U.S. Pat. No. 4,435,976 describes the use of a load cell to determine the forces applied during Brinell tests and employs a feedback loop to automatically compensate factors which affect the accuracy of the measurements, such factors being temperature and friction.
Another instance U.S. Pat. No. 6,142,010 describes the use of control closed loop systems including a load cell to measure and apply force for a Rockwell tester.
Another instance U.S. Pat. No. 6,247,356 describes the use of control closed loop systems including a load cell to measure and apply force for a micro hardness tester.
These apparatuses employ load cells as sensors to measure applied force. The weakness is they can only be used for one kind of hardness test. For example, the Rockwell tester cannot for be used for Vickers, Brinell or Micro hardness tests. Another weakness is these testers employ pancake load cells. This kind of load cell is only accurate to 0.1% to 0.5%. So, they cannot be used for measuring across a large range of forces.
U.S. Pat. No. 6,142,010 and U.S. Pat. No. 6,247,356 utilize a structure that does not provide enough support to withstand larger applied forces. This influences measurement accuracy.
All known bottom-referencing type hardness testing machines, using both load cell and deadweight style, employ an elevating screw to accommodate different specimen sizes. The mechanical forces employed in the elevating screw also can contribute to degradation of displacement measurement accuracy because of the possibility of additional deflection loss which can contribute to the inaccuracy of the displacement measurement.
All known prior hardness testers are not suitable to test holes or groove shaped specimens.